This invention relates to a method of treating the glass substrate for a liquid crystal display device, more particularly, to a method of treating the glass substrate to permit unidirectional alignment of the liquid crystal molecules on a horizontal plane.
Presently, most of liquid crystal display devices are of twisted nematic (TN) type which is advantageous in that the power consumption is small and that the device can be operated at a low voltage. In the TN type liquid crystal display device, the liquid crystal molecules are unidirectionally aligned with respect to the electrode-bearing surface of the substrate. Further, the alignment directions of the molecules are twisted by 90.degree. between the upper substrate and the lower substrate of a cell. To this end, two glass substrates each treated to permit unidirectional alignment of the liquid crystal molecules are disposed parallel with each other such that the alignment direction permitted by one of the two substrates is perpendicular to that permitted by the other substrate.
Two conventional methods are available for treating the substrate to enable the electrode-bearing surface to permit unidirectional alignment of the liquid crystal molecules, i.e., a slant vapor deposition method and a rubbing method.
The slant vapor deposition method comprises the step of depositing SiO, Pt, Au, Cu, Pb, Al, Sn, Ag, Cr, etc. on the substrate at an inclination of about 5.degree. with respect to the substrate surface. The slant vapor deposition method is advantageous in that the aligning property of the liquid crystal is not disturbed even if the substrate is exposed to high temperatures at the frit seal step. However, the requirement of a very small inclination angle of the deposition layer renders it difficult to control the vapor deposition operation when it comes to a large substrate having any of the sides exceeding 2cm. In the case of a substrate sized, for example, 10cm.times.5cm, it is necessary to dispose the means for evaporating the deposition material at a distance of more than 50cm from the substrate. Otherwise, a deposition layer of a predetermined inclination angle can not be obtained over the entire surface of the substrate, failing to perform a desired unidirectional alignment of the liquid crystal molecules.
The long distance of the evaporation means from the substrate leads to a very unfavorable productivity of the substrate, including an increased scattering, or loss, of the deposition material, an elongated deposition time and requirement of a large vapor deposition apparatus.
An additional difficulty inherent in the slant vapor deposition method is that the tilt angle, i.e., the angle made between the liquid crystal molecule and the substrate surface, is large, resulting in a low contrast and a narrow visual angle of the liquid crystal display device comprising such a substrate.
On the other hand, the rubbing method comprises the steps of forming an SiO film on the electrode-bearing surface of the substrate and rubbing the surface of the SiO film with de-fatted cotton, or the like. The substrate treated by the rubbing method is not satisfactory in durability against heat. Further, the aligning property of the liquid crystal tends to be disturbed when a cell comprising such a substrate is subjected to frit seal, resulting in a markedly short life of the liquid crystal display device. Accordingly, it was customary to employ the slant vapor deposition method for the case of involving a frit seal, in order to obtain stability and reliability of the liquid crystal display device.